1. Field of the Invention
The present invention relates to determining traveltimes for seismic pulses and more particularly to generating source and receiver traveltime tables very efficiently so that two dimensional and three dimensional prestack depth migration can be economically performed.
2. Related Prior Art
Prior art has illustrated many ways for producing traveltime tables. Traveltime tables are usually generated by shooting a fan of rays and interpolating between the rays. There were problems with critical angles. Also the method is more difficult to apply in three dimensional work.
The source and receiver traveltime tables are generated at present by shooting a fan of rays and the traveltimes are obtained by recording the traveltimes along the ray-path. The complete traveltime table is then obtained by interpolation. This method can provide traveltimes for any depth point (x,z). A prior art example of this type of ray tracing to obtain traveltime tables is illustrated in the following United States Patent.
U.S. Pat. No. 4,839,869, "Methods for Processing Converted Wave Seismic Data", issued to Chris T. Corcoran, relates to methods for processing converted wave seismic data which includes, fractional point gathering of the data in a manner consistent with a selected velocity model, dynamic correction of the data using parameters measured from the data to account for the asymmetric travel path of the converted wave rays and stacking the dynamically corrected data. Methods are also provided for updating the velocity model.